Frequency doubling in three laser regimes using nonlinear polarization rotation laser

Abstract

A unidirectional ring cavity was developed and characterized, using a Yb-doped double-clad fiber with nonlinear polarization rotation (NPR). Depending on the relative orientations of the quarter and half-wave plates, we observed that the laser operated in different regimes: Continuous-wave(CW), Q-switching(QS), and Modelocking(ML). We observed that the central wavelength was at 1064.5 nm in each of the different laser regimes. Mode-locking a laser with NPR is attractive due to its simplicity, with the locking being induced by the optical Kerr effect along with the fiber while the wave-plates and filters act as an artificial saturable absorber. The total cavity length of the Yb-doped fiber laser was about 10.8 m, including a free space segment, giving us a 18 MHz repetition rate for the mode-locked pulses. In the Q-switched regime, a pulse train with repetition rates from 14 kHz to 35 kHz and pulse widths from 980 ns to 3 μs were observed by varying the pump power. Lastly, we utilized this laser as a pump source to conduct a comparative study of the frequency-doubling characteristics in a single-pass configuration using MgO:PPLN crystal of 10 mm and 20 mm lengths in the three laser regimes. The second harmonic efficiencies obtained at CW, QS, ML for the crystal length of 10 mm were 2.15% , 11.1%, and 13.6% while for 20 mm they were 2.27%, 15.8%, and 18.7% respectively.